Seismic surveying is used for imaging subterranean sections and/or identifying subterranean elements of interest, such as hydrocarbon reservoirs, freshwater aquifers, gas injection zones, and so forth. In seismic surveying, seismic sources are activated to generate seismic waves directed into a subsurface structure.
The seismic waves generated by the seismic sources travel into the subsurface structure of the Earth, with a portion of the seismic waves being reflected back by the subsurface rock structures to the surface where they are received by seismic sensors (e.g. geophones, accelerometers, etc.). These seismic sensors produce signals that represent the detected seismic waves reflected from the subsurface structures. Signals from the seismic sensors may be processed to yield information about the content and characteristics of the subterranean structure and/or to generate images of the subsurface. To process information about the content and characteristics of the subterranean structure and/or to generate images of the subsurface, the reflected data must be analyzed with the effects of the seismic survey process, the interactions of waves with rock structures, the effects of wave transmission through the Earth structures, the effects of noise and interactions between waves all being analyzed and processed to convert the seismic signals generated by a seismic source into data providing information about the content and characteristics of the subterranean structure and/or to generate images of the subsurface.
A land-based seismic survey arrangement may include a deployment of an array of seismic sensors on the ground, which may comprise receivers distributed in a borehole and/or at the earth-air interface. A marine survey arrangement can include a seabed cable or other arrangement of seismic sensors placed on the seafloor or other water bottom surface, or can include a seismic source assembly and a seismic sensor streamer towed through a body of water.
The detected seismic waves reflected from the subsurface structures may comprise primaries, which have been reflected from the subsurface structures once, and multiples, which have been reflected from the subsurface structures more than once. The multiples may comprise free-surface multiples, which are multiples that have underside reflections at the free surface (the earth-air or water-air interface), and internal multiples, which are multiples that have underside reflections at the subsurface structures. Multiples with characteristics of both free-surface and internal multiples may also be present. Multiples may require specific processing approaches, which can comprise processing the data to a state where the multiples have been removed as noise or analyzing the multiples as signal to obtain information about the content and characteristics of the subterranean structure and/or to generate images of the subsurface.
In general, multiples have been removed from seismic data tin order to generate a seismic image. More, recently, multiples have been used as signal to generate a seismic image. However, previous seismic survey techniques have not compensated for missing recorded data when processing multiples as signal and, therefore any areal illumination gains from using the multiples as signal were limited.
Methods that use recorded free-surface multiples as signal (whether in combination with recorded primaries or not and encompassing any treatment or otherwise of internal multiples), but take no steps to compensate for missing data map some of the recorded free-surface multiples incorrectly into the subsurface, leading to cross-talk and other types of noise in the seismic image. This noise degrades seismic image quality and may be difficult to remove. Uncompensated missing recorded data limits the seismic image quality gains, e.g. in resolution or areal illumination, that could otherwise be gained from including the recorded free-surface multiples as signal. In both seismic imaging methods that do not use recorded free-surface multiples as signal, and in seismic imaging methods that do use recorded free-surface multiples as signal, whether in combination with the recorded primaries or not, the requirement of quasi-complete data, that is, good coverage by both shots and receivers, also affects the design of seismic surveys to acquire such data.